U.S. patent application number 14/039665 was filed with the patent office on 2014-08-21 for apparatus for coherent beam combining in an array of laser collimators.
This patent application is currently assigned to U.S. Army Research Laboratory ATTN: RDRL-LOC-I. The applicant listed for this patent is U.S. Army Research Laboratory ATTN: RDRL-LOC-I. Invention is credited to Leonid A. Beresnev, Gary W. Carhart, Jony J. Liu.
Application Number | 20140231618 14/039665 |
Document ID | / |
Family ID | 51350504 |
Filed Date | 2014-08-21 |
United States Patent
Application |
20140231618 |
Kind Code |
A1 |
Beresnev; Leonid A. ; et
al. |
August 21, 2014 |
Apparatus for Coherent Beam Combining in an Array of Laser
Collimators
Abstract
A method and apparatus for coherent beam combining in an array
of laser beam collimators. The array of laser beam collimators
includes an array of a plurality collimating lenses, each lens
intercepting a respective one of a plurality of divergent laser
beams. Each collimating lens is joined with adjacent collimating
lenses such that an output aperture is formed with a common vertex
of the adjacently joined collimating lenses. A concave mirror is
positioned a distance from the common vertex for receiving a
fraction of each of the collimated laser beams that passed through
a portion of each of the collimating lenses that are adjacent to
the common vertex, and then providing reflected fractional
collimated laser beams. A sensor intercepts the reflected
fractional collimated laser beams so as to provide a signal that is
applied to synchronize the phase of each of the collimated laser
beams.
Inventors: |
Beresnev; Leonid A.;
(Columbia, MD) ; Liu; Jony J.; (Olney, MD)
; Carhart; Gary W.; (Elkton, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
U.S. Army Research Laboratory ATTN: RDRL-LOC-I |
Adelphi |
MD |
US |
|
|
Assignee: |
U.S. Army Research Laboratory ATTN:
RDRL-LOC-I
Adelphi
MD
|
Family ID: |
51350504 |
Appl. No.: |
14/039665 |
Filed: |
September 27, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61709209 |
Oct 3, 2012 |
|
|
|
Current U.S.
Class: |
250/201.9 |
Current CPC
Class: |
H01S 3/2383 20130101;
H01S 3/005 20130101; G02B 27/0087 20130101; H01S 3/1307
20130101 |
Class at
Publication: |
250/201.9 |
International
Class: |
H01S 3/13 20060101
H01S003/13; G02B 27/09 20060101 G02B027/09 |
Goverment Interests
GOVERNMENT INTEREST
[0002] Governmental interest--The invention described herein may be
manufactured, used and licensed by or for the U.S. Government.
Claims
1. Apparatus for coherent beam combining in an array of laser beam
collimators, comprising: an array of a plurality collimating
lenses, each lens intercepting a respective one of a plurality of
divergent laser beams, wherein each collimating lens is joined with
adjacent collimating lenses such that an output aperture is formed
with a common vertex of the adjacently joined collimating lenses; a
concave mirror positioned a distance from the common vertex for
receiving a fraction of each of the collimated laser beams that
passed through a portion of each of the collimating lenses that are
adjacent to the common vertex, and providing reflected fractional
collimated laser beams; and a sensor for intercepting the reflected
fractional collimated laser beams so as to provide a signal that is
applied to synchronize the phase of each of the collimated laser
beams.
2. The apparatus of claim 1 further including a a plurality of mask
elements, each mask element having a shape adapted for clipping of
periphery areas of a respective one of each of the divergent laser
beams so as to pass a substantial portion of the Gaussian profile
of the divergent laser beam therethrough and toward a
correspondingly shaped one of the collimating lenses.
3. The apparatus of claim 2, wherein the shape of the mask elements
and the collimating lenses is hexagonal.
4. The apparatus of claim 2, wherein the mask elements clip
substantially 100% of the Gaussian profile of the divergent laser
beams that pass therethrough.
5. The apparatus of claim 1, wherein the concave mirror is
positioned so as to reflect the fractional collimated laser beams
through the vertex to a sensor positioned on a side of the array
opposite the output aperture of the array.
6. The apparatus of claim 1, wherein the concave mirror is
positioned so as to reflect the fractional collimated laser beams
to a sensor positioned adjacent to the output aperture of the
array.
7. The apparatus of claim 1, wherein the sensor comprises a pinhole
and photodiode assembly.
8. The apparatus of claim 1, wherein the sensor comprises an imager
having a plurality of photosensor elements that simulate a pinhole
and photodiode assembly.
9. The apparatus of claim 4, wherein the mask elements comprise a
node of a plurality of partly-reflecting-partly-absorbing mirrors,
each mirror for intercepting periphery areas of a respective one of
the divergent laser beams.
10. The apparatus of claim 9, wherein the mask elements further
comprise a node of a plurality of radiation traps, each radiation
trap receiving a periphery area of the divergent laser beam that is
reflected by a corresponding one of the
partly-reflecting-partly-absorbing mirrors.
11. A method for phase-locking a plurality of coherent laser beams
in an array of laser beam collimators, comprising: providing an
array of plurality of adjacently positioned laser beam sources for
projecting a corresponding plurality of divergent laser beams
having a Gaussian profile along a corresponding plurality of
predetermined paths; positioning a respective one of a plurality of
collimating lenses in the predetermined path of each of a
respective corresponding one of the divergent laser beams so as to
provide at an output of each collimating lens a corresponding
collimated laser beam, and wherein each collimating lens is joined
with adjacent collimating lenses such that an output aperture of
the array is formed with a common vertex of the adjacently joined
collimating lenses; positioning a concave mirror a distance from
the output aperture so as to receive a fraction of each of the
collimated laser beams that passed through a portion of each of the
collimating lenses that are adjacent to the common vertex, and
provide reflected fractional collimated laser beams; and
intercepting the reflected fractional collimated laser beams with a
sensor so as to provide a signal that is applied to synchronize the
phase of the laser beam sources that provided the fractional
collimated laser beams.
12. The method of claim 11, where positioning the collimating
lenses comprises positioning respective ones of the collimating
lenses so that they receive a substantial portion of the Gaussian
beam profile of a corresponding one of the divergent laser
beams.
13. The method of claim 12, wherein said substantial portion is in
the range from 90 to 100%.
14. The method of claim 12, further including positioning in the
predetermined path of each divergent laser beam a corresponding one
of a plurality of mask elements, each mask element having a shape
adapted for clipping of periphery areas of a respective one of each
of the divergent laser beams so as to pass a substantial portion of
the Gaussian profile of the divergent laser beam therethrough and
toward a correspondingly shaped one of the collimating lenses,
thereby causing said substantial portion to be substantially
100%.
15. The method of claim 11, where the shape of the collimating
lenses is hexagonal.
16. The method of claim 11 where positioning a concave mirror
comprises positioning of the concave mirror so as to reflect the
fractional collimated laser beams through the common vertex of the
output aperture to a sensor positioned on a side of the array
opposite the output aperture of the array.
17. The method of claim 11, where positioning a concave mirror
comprises positioning of the concave mirror so as to reflect the
fractional collimated laser beams to a sensor positioned adjacent
to the output aperture of the array.
18. The method of claim 11, where intercepting comprises using as
the sensor a pinhole and photodiode assembly.
19. The method of claim 11, where intercepting comprises using as
the sensor an imager having a plurality of photosensor elements
that simulate a pinhole and photodiode assembly.
20. The method of claim 14, positioning a corresponding one of a
plurality of mask elements comprises positioning a node of a
plurality of partly-reflecting-partly-absorbing mirrors, each
mirror for intercepting periphery areas of a respective one of the
divergent laser beams, and positioning a node of a plurality of
radiation traps, each radiation trap receiving a periphery area of
the divergent laser beam that is reflected by a corresponding one
of the partly-reflecting-partly-absorbing mirrors.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. provisional patent
application Ser. No. 61/709,209, filed Oct. 3, 2012, which is
herein incorporated by reference.
FIELD OF INVENTION
[0003] Embodiments of the present invention generally relate to
coherent laser beam combining in an array of laser beam collimators
and, more particularly, to a method and apparatus for phase-locking
of a plurality of laser beams in the array.
BACKGROUND OF THE INVENTION
[0004] One known Coherent Beam Combining system (CBC) is based on a
sparse multi-aperture array of fiber optic collimators. As a metric
for external active control of phase-locking a technique called the
power in the bucket (PIB) is used The PIB technique uses the
intensity of the photons returned from a target for feedback
control of phase shifters that control the phase of the laser beam
sources. FIG. 1 illustrates such a system.
[0005] The travel time of the return photons from the target is
variable due to reliance of target reflection and atmospheric
conditions, which may cause time delays that prevent optimization
of the phase locking performance.
[0006] Preliminary phase locking of the source beams can solve
these drawbacks using for instance beam splitters in the train of
output laser beams, as shown in FIG. 2. Here, the CBC system with
external phase-locking uses portions of beamlets split from the
power train by means of beam splitters placed in a near-field of
the laser beam power train. The overlapping of these beamlet
portions near the focal plane of a focusing lens yields
constructive interference spots, at least one of which is selected
by use of a pinhole. A photo-sensor placed behind the pinhole is
used to indicate an intensity for the selected spot, which
intensity is used as a metric for active feedback control of the
phase of the source beams. However, use of beam splitters which are
external to the fiber collimators are disadvantageous in that they
can cause wave front power aberrations in the laser beams, they are
bulky, and delicate elements make the system heavy and
non-reliable, especially for mobile applications.
[0007] FIGS. 3A and 3B show side and end schematic views of an
arrangement that uses the internal photons of constituent laser
beams, that is, before the photons reach the output collimating
lenses. These photons are intercepted in periphery areas of the
divergent (Gaussian) beams, which are parasitic beam-tails that
remain inside of the beam array. More specifically, for internal
phase-locking of wave fronts of an array of fiber optic
collimators, the periphery areas of Gaussian beams (i.e., beam
tails) are used, which beam tails, in one embodiment, are clipped
before reaching the output collimating lenses. FIG. 3A illustrates
the internal phase-locking of neighbor fiber optic collimators 101
and 102 using beam tails 110-2 and 120-1 of Gaussian beams 110 and
120. Mirrors or diffractive optic elements (DOE) 600-1 and 600-2
intercept/clip the beam tails 110-2 and 120-1 and re-direct them to
the back of the array where they are focused on a plane near a
pinhole photodetector. The pinhole selects the constructive
interference spot of these beams after their overlap near the focal
plane. An intensity signal from the photodiode placed behind of the
pinhole provides a metric for internal feedback phase-locking of
neighbor beamlets. A simplest example includes two channels as
shown in FIG. 3A. In the case of hexagon packing of sub-apertures,
instead of two, three mirrors 600-1, 600-2, 600-3 are used to
intercept the beam tails of three adjacent sub-apertures 210, 220,
230.
[0008] Drawbacks of the internal phase-locking: [0009] Diffractive
optic elements (DOE) or precision assembly of three parabolic
sub-mirrors or mini-holograms (in case of hexagonal beamlet
packaging) intercepting the Gaussian beam tails, are complicated
and expensive optical devices. [0010] DOEs or assemblies of
sub-mirrors are inside of array, causing the problems of precision
alignment and possible thermal aberration in case of high beams
power. [0011] The size of interference spot behind of array is very
small (typically 5-15 microns), causing the problem of alignment
and stability of the pinhole with small diameter.
BRIEF SUMMARY OF THE INVENTION
[0012] A method and apparatus for coherent laser beam combining in
an array of laser beam collimators as shown in and/or described in
connection with at least one of the figures, as set forth more
completely in the claims.
[0013] These and other features and advantages of the present
disclosure may be appreciated from a review of the following
detailed description of the present disclosure, along with the
accompanying figures in which like reference numerals refer to like
parts throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] So that the manner in which the above recited features of
the present invention can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments.
[0015] FIG. 1 illustrates a known Coherent Beam Combining system
(CBC) based on sparse multi-aperture array of fiber optic
collimators;
[0016] FIG. 2 illustrates a known CBC system including being
splitters in the train of output laser beams;
[0017] FIGS. 3A and 3B illustrate side and end schematic views of
an arrangement for internal phase-locking of wave fronts of an
array of fiber optic collimators using the periphery areas of
Gaussian beams (beam tails), which are clipped by output
lenses;
[0018] FIG. 4 illustrates a schematic view of one embodiment of
method and apparatus for coherent beam combining in an array of
laser beam collimators;
[0019] FIG. 5 illustrates a schematic view of another embodiment of
method and apparatus for coherent beam combining in an array of
laser beam collimators that is similar to FIG. 4, but includes 2
external feedback loops;
[0020] FIG. 6 illustrates a schematic view of another embodiment of
a method and apparatus for coherent beam combining in an array of
laser beam collimators that is similar to FIG. 4, but where the
micro-mirror re-directs the sub-beams to a sensor located outside
of the array rather than inside of the array.
[0021] FIG. 7 illustrates a rear perspective view of the FIG. 4
embodiment;
[0022] FIG. 8 illustrates a schematic view of another embodiment of
a method and apparatus for coherent beam combining in an array of
laser beam collimators that is similar to FIG. 4, where the
micro-lens is a positive lens;
[0023] FIG. 9 illustrates a schematic view of another embodiment of
a method and apparatus for coherent beam combining in an array of
laser beam collimators that is similar to FIG. 4, where the
micro-lens is a negative lens;
[0024] FIG. 10 illustrates a schematic view of another embodiment
of a method and apparatus for coherent beam combining in an array
of laser beam collimators that is similar to FIG. 4, and includes
details of mask apparatus for the interception and dissipation of
power of the periphery areas of the Gaussian beams;
[0025] FIG. 11 illustrates a schematic view of another embodiment
of a method and apparatus for coherent beam combining in an array
of laser beam collimators that is similar to FIG. 4, and includes
superstrates for supporting the collimator lenses and micro
mirrors;
[0026] FIG. 12 illustrates an evaluation of power conditions for
the micro-mirror embodiments described in the prior Figures;
[0027] FIG. 13 illustrates a schematic view of another embodiment
of a method and apparatus for coherent beam combining in an array
of laser beam collimators that is similar to FIG. 4, but where no
mask is used to pre-shape the laser beams so as to match the
collimator lenses; and
[0028] FIG. 14 illustrates a schematic view of a transport
superstrate for accommodating the micro-mirror shown in FIG. 6 and
FIG. 13.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Embodiments of the present invention generally relate to
coherent laser beam combining in an array of laser beam collimators
and, more particularly, to a method and apparatus for coherent beam
combining in an array by providing an array of a plurality of laser
beams and an array of a corresponding plurality of adjacently
joined collimating lenses, each lens intercepting a respective
laser beam so as to provide at an output side of the each
collimating lens a collimated laser beam, wherein each collimating
lens is joined with adjacent collimating lenses such that an output
aperture is formed with a common vertex of the adjacently joined
collimating lenses. A concave mirror positioned a distance from the
common vertex so as to receive a fraction of each of the collimated
laser beams that passed through a portion of each of the
collimating lenses that are adjacent to the common vertex, and
reflect said fractional collimated laser beams. A sensor intercepts
the reflected fractional collimated laser beams so as to provide a
signal that is applied to synchronize the phase of each of the
collimated laser beams.
[0030] In a further embodiment parasitic radiation of the divergent
beams are intercepted by a mask positioned before the collimating
lenses, the mask comprising a combination of Partially
Reflecting-Partially Absorbing (PR-PA) plates which dissipate the
intercepted parasitic radiation by means of external cooling.
[0031] In a further embodiment an optical sensor measures the
photons returned from the target and a motorized displacement of
the optical sensor aligns the non-common path differences of wave
fronts of the laser beams to modulo 2.pi.. In one embodiment, the
optical sensor comprises a pin hole and a photodiode detector. In
an alternative embodiment, a two dimensional imager comprises the
optical sensor, and the size and position of the imaging area is
adjusted so as to simulate the pin hole, the photodiode detector
and an X-Y movement of the photodiode detector.
[0032] More particularly, embodiments of the present invention:
[0033] a) provide an array of plurality of adjacently positioned
laser beam sources, each source projecting along a predetermined
path a divergent laser beam having a Gaussian profile; [0034] b)
position in the predetermined path of each divergent laser beam a
corresponding one of a plurality of mask elements, each mask
element having a shape adapted for clipping periphery areas of the
divergent laser beam passing therethrough so as allow a substantial
portion of the Gaussian profile of the beam to continue on its
predetermined path; [0035] c) position a respective one of a
corresponding plurality of collimating lenses in the predetermined
path of each divergent laser beam after said beam has passed
through a corresponding mask element, wherein the shape of each
collimating lens corresponds substantially with the shape of the
corresponding mask elements and the positioning of the collimating
lenses are such that each collimating lens intercepts a substantial
portion of the Gaussian profile of the corresponding divergent
laser beam which passed through a corresponding mask element (such
as 100% of the beam profile), so as to provide at an output
aperture of each collimating lens a corresponding collimated laser
beam, and wherein each collimating lens is joined with adjacent
collimating lenses such that an output aperture of the array is
formed with a common vertex of the adjacently joined collimating
lenses; [0036] d) position a concave mirror a distance from the
common vertex for receiving a fraction of each of the collimated
laser beams that passed through a portion of each of the
collimating lenses that are adjacent to the common vertex, and
providing reflected fractional collimated laser beams; and [0037]
e) intercept the reflected fractional collimated laser beams with a
sensor so as to provide a signal that is applied to synchronize the
phase of the laser beam sources that provided the fractional
collimated laser beams.
[0038] In an alternative embodiment, the mask elements are not
used, and instead the array of collimating lenses are positioned so
as to intercept the Gaussian profile of the divergent laser beam so
as to have a substantial fill factor, such as 90% or greater, and
preferably 95% or greater. In such an embodiment, the parasitic
radiation passing through the adjacent lenses is shielded by use of
a heat dissipating tube surrounding the output aperture of the
array and having a diameter slightly larger than output aperture of
array.
[0039] Various embodiments of a method and apparatus for coherent
laser beam combining in an array of laser beam collimators are
described. In the following detailed description, numerous specific
details are set forth to provide a thorough understanding of
claimed subject matter. However, it will be understood by those
skilled in the art that claimed subject matter may be practiced
without these specific details. In other instances, methods,
apparatuses or systems that would be known by one of ordinary skill
have not been described in detail so as not to obscure claimed
subject matter.
[0040] Some portions of the detailed description that follow are
presented in terms of algorithms or symbolic representations of
operations on binary digital signals stored within a memory of a
specific apparatus or special purpose computing device or platform.
In the context of this particular specification, the term specific
apparatus or the like includes a general-purpose computer once it
is programmed to perform particular functions pursuant to
instructions from program software. Algorithmic descriptions or
symbolic representations are examples of techniques used by those
of ordinary skill in the signal processing or related arts to
convey the substance of their work to others skilled in the art. An
algorithm is here, and is generally, considered to be a
self-consistent sequence of operations or similar signal processing
leading to a desired result. In this context, operations or
processing involve, physical manipulation of physical quantities.
Typically, although not necessarily, such quantities may take the
form of electrical or magnetic signals capable of being stored,
transferred, combined, compared or otherwise manipulated. It has
proven convenient at times, principally for reasons of common
usage, to refer to such signals as bits, data, values, elements,
symbols, characters, terms, numbers, numerals or the like. It
should be understood, however, that all of these or similar terms
are to be associated with appropriate physical quantities and are
merely convenient labels. Unless specifically stated otherwise, as
apparent from the following discussion, it is appreciated that
throughout this specification discussions utilizing terms such as
"processing," "computing," "calculating," "determining" or the like
refer to actions or processes of a specific apparatus, such as a
special purpose computer or a similar special purpose electronic
computing device. In the context of this specification, therefore,
a special purpose computer or a similar special purpose electronic
computing device is capable of manipulating or transforming
signals, typically represented as physical electronic or magnetic
quantities within memories, registers, or other information storage
devices, transmission devices, or display devices of the special
purpose computer or similar special purpose electronic computing
device.
[0041] FIG. 4 illustrates a schematic view of one embodiment of
method and apparatus for coherent beam combining in the array of
laser beam collimators.
[0042] Three neighbor lasers are shown as illustrative of a
scalable unit in an array having a hexagon arrangement of
sub-apertures. The scalable unit arrangement includes fiber lasers
100-1, 100-2 and 100-3 and output collimator lenses 200-1, 200-2
and 200-3 arranged so as to have in combination with mask clipping
elements 300 an approximately 100% fill factor, thereby avoiding
parasitic propagation of periphery areas of the beams. A vertex
200-123 comprises an opening formed at the corners of the three
adjacent lens 200-1, 200-2 and 200-3. The opening 200-123 has a
characteristic size d. A focusing micro-lens 300-123 having a
diameter d is positioned in opening 200-123. In this embodiment a
clipping mask 300-1 is illustrated as useful for clipping the
divergent beam supplied by laser 100-1 to a shape which matches the
shape of the corresponding collimator lens 200-1, resulting in
substantially 100% filling of lens 200-1. In practice, a mask 300
would be provided for each lens 200. A spherical concave mirror
400-123 having a diameter D>d is positioned outside of the array
and in alignment with the opening in the vertex. A key part of
these embodiments are sub-areas 200-1C, 200-2C, 200-3C of lenses
200-1, 200-2, 200-3 near the vertex, which provide three collimated
sub-beams 1C, 2C, 3C (sub-beam 3C is not shown). Hereinafter the
collimated sub-beams 1C, 2C, 3C are also referred to as fractional
beams 1C, 2C and 3C. A spherical micro-mirror 400-123 re-directs
the sub-beams 1C, 2C and 3C to a sensor for developing a signal
that is applied to synchronize the phase of each of the collimated
laser beams. In this embodiment, the micro-mirror 400-123
re-directs the sub-beams 1C, 2C and 3C to the back of array via a
focusing micro-lens 300-123.
[0043] Micro-mirror 400-123 and micro-lens 300-123 essentially form
a telescope that provides overlapping of sub-beams 1C, 2C, 3C in
between lasers 100-1, 100-2, 100-3, preferably on a focal plane
positioned behind the lasers, and on which focal plane an
interference pattern of sub-beams 1C, 2C and 3C occurs. In this
embodiment a pinhole-photodetector assembly 500-123 is positioned
for selecting the constructive interference spot of the
interference pattern and providing a signal to control a processor
(such pinhole-photodetector assembly and processors are well known
to those of ordinary skill in this, art) which synchronizes (phase
locks) the phases of the sources for the three output beams 1, 2, 3
(beam 3 is not shown).
[0044] Note, in an alternative embodiment to the FIG. 4
arrangement, such as shown in FIGS. 13 and 14, the sub-beams 1C, 2C
and 3C are re-directed outside of the array instead of through
it.
[0045] It is also noted that in a further alternative embodiment to
the FIG. 4 arrangement, because of the small angle between
re-directed sub-beams 1C, 2C and 3C (small in comparison to the
large angle between the reflected beamlets shown in the FIG. 3A
arrangement), sensor 500-123 could be a "synthetic
pinhole-photodiode", that is, a plurality of photosensors formed
electronically by computer control from an array of photosensitive
pixels (such as an array of CCD, CMOS target, micro-bolometer, etc,
elements). The shape and size of plurality of photosensitive
elements that simulate the pinhole-photodiode can be adjusted by
the computer to optimize the performance of the phase locking. In
the described embodiments having small angles between the
re-directed sub-beams (as compared with the large angles in
embodiments such as shown by FIG. 3A), the characteristic size of
the interference spot at the same dimensions of an array (such as
the overall dimensions of the array shown in the FIG. 3A
arrangement), can be 10 times larger than in case of the FIG. 3A
arrangement. Thus, instead of requiring use of a pinhole having a
diameter as small as 10-15 microns, the "synthetic
pinhole-photodiode" plurality of photosensors having an overall
diameter 100-150 microns can be formed using a CCD target, for
example, with a modest pixel size of 15-20 microns, which allows
for reliable computer control of the position of the selected
plurality of elements. Such computer control of the plurality
thereby provides for the electronic adjustment of the non-common
phase difference to modulo 2.pi. between adjacent sub-apertures.
This particular alternative is not specifically shown in the
Figures and element 500-123 is representative thereof.
[0046] Thus, as described above, the output aperture in these
embodiments are composed from densely packed hexagon lenses (where
lenses 200-1, 200-2 and 200-3 are illustrative of a scalable unit)
with almost 100% fill factor, In an alternative embodiment, square
lenses can be used for rectangular packing of beamlets, however the
hexagon lenses and corresponding honeycomb arrangement of beamlets
has better CBC performance. The collimating lenses can be attached
together on a transparent superstate, as described below in
conjunction with FIG. 11, or attached to each other via adhesion
(such as using glue or other bonding techniques) along their
respective edges.
[0047] The summarized output beam leaving the array is formed with
densely packed collimated beams, each having a hexagonal
cross-section.
[0048] The set of concave micro-mirrors 400-123 is placed outside
of the output aperture, with the center of each micro-mirror
coinciding with the common vertex of three adjacent collimator
lenses, such as lenses 200-1, 200-2 and 200-3. In a scaled
arrangement, the multiple micro-mirrors can be attached to a second
transparent superstate, as also described below in conjunction with
FIG. 11.
[0049] The corners of these of the three lenses 200-1, 200-2 and
200-3 are cut so as to provide the opening d of vertex 200-123 to
be significantly less than the diameter D of micro-mirror
400-123.
[0050] The micro-lens 200-123 is placed into the opening at a
position so as to obtain the highest possible fill factor. In the
illustrated embodiment, the micro-lens 200-123 is considered as a
round concave of convex spherical lens.
[0051] The micro-mirror 400-123 intercepts the fractions
(sub-beams) of the three collimated beams 1C, 2C and 3C in the
vicinity of the hole and reflects these three sub-beams back to the
micro-lens.
[0052] The micro-mirror 400-123 is coupled optically with the
micro-lens 200-123 and forms a telescopic system that provides
focusing of the three sub-beams behind the array, preferably behind
the laser sources. At the plane where these three sub-beams are
overlapping, the constructive interference is formed.
[0053] The pinhole of the pinhole-photodetector assembly 500-123
selects the spot of the interference pattern and the photodetector
behind of pinhole provides the signal for phase locking of the
three output beams.
[0054] The pinhole-photodetector assembly is placed on a movable
platform. X-Y displacement (manual or computerized) of the movable
platform provides the control of the non-common path difference
between wave fronts of three sub-apertures to tune this difference
to modulo 2.pi..
[0055] In an embodiment having the fore mentioned "synthetic
pinhole-photodiode" the plurality of photosensor elements are
selected by computer control so as to electronically move this
plurality (and thereby simulate the pinhole and fore noted X-Y
displacement) so as to match the non-common phase difference
between the adjacent sub-apertures to modulo 2.pi..
[0056] The mask positioned in-between the laser sources and the
output lenses provides the clipping of periphery areas of the
respective divergent beams so as to avoid parasitic illumination of
neighbor lenses. This mask intercepts and dissipates the parasitic
power in these intercepted beam tails.
[0057] FIG. 5 illustrates a schematic view of another embodiment of
method and apparatus for coherent beam combining in the array of
laser beam collimators that is similar to FIG. 4, but includes 2
feedback loops.
[0058] Loop 1: mirror 400-123, micro-lens 300-123,
pinhole-photodetector assembly 500-123, provide the input signal
for phase-locking of beams 1, 2, 3 (beam 3 not shown) through the
use of the phase shifter processor 600-123.
[0059] Loop 2: photo-receiver 700 receives the photons 505 returned
by the target, for providing the input for control of the X-Y
position of the pinhole-photodetector assembly 500-123 for
optimization of non-common path differences between wave fronts of
beams 1, 2 and 3.
[0060] FIG. 6 illustrates a schematic view of another embodiment of
a method and apparatus for coherent beam combining in an array of
laser beam collimators that is similar to FIGS. 4 and 5, but where
the micro-mirror 400-123 re-directs the sub-beams to a sensor
located outside of the array rather than inside of the array. Thus,
as shown in FIG. 6, micro-mirror 400-123, representative of a
scalable unit for three sub-beams in the arrangement, re-directs
the sub-beams 1C, 2C and 3C to a sensor 500-123 positioned outside
of the array, and specifically at an outer edge thereof. FIGS. 13
and 14, described below, show additional details of such an
arrangement.
[0061] FIG. 7 illustrates a rear perspective view of the FIG. 4
embodiment for further illustration of the FIG. 4 embodiment. No
further description is deemed necessary.
[0062] FIG. 8 illustrates a schematic view of another embodiment of
a method and apparatus for coherent beam combining in the array of
laser beam collimators that is similar to FIG. 4, where micro-lens
300-123 is a positive lens. No further description is deemed
necessary.
[0063] FIG. 9 illustrates a schematic view of another embodiment of
a method and apparatus for coherent beam combining in the array of
laser beam collimators that is similar to FIG. 4, where micro-lens
300-123 is a negative lens. No further description is deemed
necessary.
[0064] FIG. 10 illustrates a schematic view of another embodiment
of a method and apparatus for coherent beam combining in the array
of laser beam collimators that is similar to FIG. 4, and is useful
for explaining details of the mask apparatus for the interception
and dissipation of power of the periphery areas of the divergent
beams.
[0065] A mask assembly comprises nodes 810 and 820 (corresponding
to one embodiment of the masks 300 of FIGS. 4 and 5). Node 810
comprises partly-reflecting-partly-absorbing mirrors 810-1, 810-2,
810-3, 810-4, for intercepting periphery areas of the respective
divergent beams from fiber lasers 100-1 and 100-2. Part of the
intercepted power is absorbed in node 810, and part of the power is
reflected to node 820, which includes radiation traps 820-1, 820-2,
820-3, 820-4. Nodes 810 and 820 are supplied with inside channels
(not shown) which includes circulating gas or liquid for conducting
and disposing the parasite heat generated by the trapping and
reflection of the radiation, to an area outside of the array (not
shown).
[0066] FIG. 11 illustrates a schematic view of another embodiment
of a method and apparatus for phase-locking of a plurality of laser
beams in the array of laser beam collimators that is similar to
FIG. 4, and includes a transparent superstrate 200-S for supporting
the output lenses 200-1. 200-2, 200-3 (lens 200-3 not shown) and
focusing lens 300-123, and a transparent superstrate 400-S
supporting the mirror 400-123.
[0067] In a test-bed for investigating the performance of coherent
beam combining for arrays up to 19 channels, the distance between
output sub-apertures was 37 mm. In an embodiment for 100% fill
factor, the lenses also had a hexagon shape with a size of 37 mm.
The mode field diameter of the operating fiber lasers was about 7
.mu.m and the optimum focal length of lenses was 174 mm.
[0068] Evaluation of power conditions for micro-mirror embodiments
is now described in conjunction with FIG. 12 illustrating mirror
400-123 and microlens 300-123. The full power incident on a
micro-mirror 400-123 having a diameter D=8 mm was determined at
distance L=174 mm to be equal to 0.16 mW. If the effective diameter
d of the vertex 200-123 shown in FIG. 4 is equal to 4 mm, the
effective power on the mirror is reduced by 0.04 mW and is equal
0.16-0.04=0.12 mW. this is a small fraction of total power 19 mW:
0.12 mW/19 mW=0.6%. At a power of 1 kW per fiber laser channel, the
full power at the mirror will be 3 kW. The fraction of power which
will hit the micro-mirror can be estimated as 19 W, which power
needs to be dissipated, for example by cooling of the
micro-mirror.
[0069] Considering a small decrease of the diameter of the
micro-mirror to D=6 mm, the intercepted power decreases to about
two times less and is estimated to be as low as 9 W. Considering a
gold reflective coating, the absorbed power will be only about 2%,
that is heating with about 200 mW is expected, which can be easily
handled with a modest cooling of the micro-mirror.
[0070] Another solution would be to use a semitransparent
reflecting coating for the micro-mirrors, for instance a dielectric
mirror having a small reflecting coefficient or a very thin
semi-transparent metal film. The excess radiation will pass though
the mirror and will be dissipated outside of the array. The
dissipated radiation of the passed beam will be on the order of
units of Watt and is not focused or collimated and is not a
significant threat to ambient at distances of tens of meters from
array. For security, a simple blending tube, positioned around the
power beam will totally solve the problem of this parasitic
scattering.
[0071] FIG. 13 illustrates a schematic view of another embodiment
of a method and apparatus for coherent beam combining in an array
of laser beam collimators that is similar to FIG. 4, but where no
mask is used to pre-shape the laser beams so as to match the
collimator lenses. Thus, the full Gaussian beams are allowed to hit
the plurality of densely packed lenses. In such an embodiment about
95% of each Gaussian beam fills the corresponding output lens, and
about 5% becomes parasitic radiation. Additionally, reflecting by
the micro-mirror 400-123 is to a sensor 500-123 positioned adjacent
to the array as shown in FIG. 6), rather than through the array. A
mount 1302 is provided to support the optical sensor 500-123 at the
appropriate position adjacent to the output aperture of the array.
Because no mask elements are provided, a parasitic portion from
each of the laser beams will pass through a neighboring,
non-corresponding collimating lens and not be reflected by a
corresponding micro-mirror. For example, the parasitic beam from
laser 1 that passes through lens 200-2 is illustrated as 1P-2, the
parasitic beam from laser 2 that passes through lens 200-1 is
illustrated as 2P-1 and the parasitic beam from laser 2 that passes
through lens 200-3 is illustrated as 2P-3. Due to the size and
positioning of the various micro-mirrors 400-123, none of these
parasitic beams are reflected and are undesirably included in the
output aperture. Accordingly, a tube 1304 having a diameter
slightly larger than the output aperture is attached to mount 1302
and extends in the direction of the propagation of the collimated
laser beams by an appropriate amount sufficient to reflect and/or
absorb these parasitic beams. In this regard, a reflective and/or
absorptive layer 1306 is provided on the interior surface of tube
1304.
[0072] FIG. 14 illustrates a schematic view of a transport
superstate for accommodating the micro-mirror shown in FIG. 6 and
FIG. 13. A superstrate 1402 having an antireflective coating is
positioned across the diameter of mount 1302. An appropriately
positioned hole 1404 in superstrate 1402 supports a post 1406 which
positions micro-mirror 400-123 at the appropriate position.
Additional holes and posts are provided, not specifically shown,
for supporting additional micro-mirrors in the arrangement.
[0073] Advantages of the disclosed embodiments: [0074] Bulky beam
splitters are not needed to lock the phases of all fiber laser
beams before they propagate the tactical distance. [0075] The power
in the vertex of three adjacent lenses is small as units of Watts
at laser beam powers in the kW range, owing to strong decay of
intensity of the divergent beams at a distance far from their
central axes. [0076] The micro-mirrors and micro-lenses used for
intercepting the fractional beams and reflecting/refocusing them
back to a sensor, either through or adjacent to the array, have a
much lower cost than the diffractive optic elements used in the
prior art techniques, and do not require special precautions for
alignment and heat dissipation. [0077] The substantial fill factors
achieved with the disclosed embodiments, such as 100% when a mask
is used and 95% in an embodiment without a mask, achieves more than
15% more power (for a 7 channel array) in beam combination than
current arrays having circular lenses.
[0078] The foregoing description, for purpose of explanation, has
been described with reference to specific embodiments. However, the
illustrative discussions above are not intended to be exhaustive or
to limit the invention to the precise forms disclosed. Many
modifications and variations are possible in view of the above
teachings, as noted several places in the above descriptions. The
embodiments were chosen and described in order to best explain the
principles of the present disclosure and its practical
applications, to thereby enable others skilled in the art to best
utilize the invention and various embodiments with various
modifications as may be suited to the particular use
contemplated.
[0079] The methods described herein may be implemented in software,
hardware, or a combination thereof, in different embodiments. In
addition, the order of methods may be changed, and various elements
may be added, reordered, combined, omitted, modified, etc. All
examples described herein are presented in a non-limiting manner.
Various modifications and changes may be made as would be obvious
to a person skilled in the art having benefit of this disclosure.
Realizations in accordance with embodiments have been described in
the context of particular embodiments. These embodiments are meant
to be illustrative and not limiting. Many variations,
modifications, additions, and improvements are possible.
Accordingly, plural instances may be provided for components
described herein as a single instance. Boundaries between various
components, operations and data stores are somewhat arbitrary, and
particular operations are illustrated in the context of specific
illustrative configurations. Other allocations of functionality are
envisioned and may fall within the scope of claims that follow.
Finally, structures and functionality presented as discrete
components in the example configurations may be implemented as a
combined structure or component. These and other variations,
modifications, additions, and improvements may fall within the
scope of embodiments as defined in the claims that follow.
[0080] Various elements, devices, modules and circuits are
described above in associated with their respective functions.
These elements, devices, modules and circuits are considered means
for performing their respective functions as described herein.
While the foregoing is directed to embodiments of the present
invention, other and further embodiments of the invention may be
devised without departing from the basic scope thereof, and the
scope thereof is determined by the claims that follow.
* * * * *